The present invention relates to a control system for an unmanned carrier vehicle, and more particularly to a steering angle control and a velocity control for an unmanned carrier vehicle.
Recently, the developed factory automation (FA) system frequently employs an automatic carrier system using the unmanned carrier vehicle. Normally, this unmanned carrier vehicle is called "automated guided vehicle", i.e., "AGV".
According to the steering angle control for the above-mentioned unmanned carrier vehicle, a sensor equipped in the vehicle detects a deviation from a guiding line (i.e., route cable) laid on its traveling way (e.g., floor), thereby controlling the steering angle such that the detected deviation will be eliminated.
FIGS. 1 and 3 respectively illustrate the same example of unmanned carrier vehicle 1 having three wheels. Herein, FIG. 1 illustrates the vehicle 1 which travels in forward direction, while FIG. 3 illustrates the vehicle 1 which travels in backward direction.
Detection of the deviation of the vehicle, to be traveled in forward direction (hereinafter, simply referred to as "forward travel"), is made by a steering sensor 7a which is directly attached with respect to a steering axis, while detection of the deviation of the vehicle, to be traveled in backward direction (hereinafter, simply referred to as "backward travel"), is made by another steering sensor 7b which is provided in a rear portion of the vehicle. Incidentally, traveling and steering operations in the forward and backward travels are made by a front wheel 2.
In FIG. 1, 3a designates a steering motor which sets the steering angle of the front wheel 2. In addition, 4 designates a traveling motor which drives the front wheel 2, and 5 designates a control box which controls the steering motor 3a based on an output signal of the steering sensor 7a. As shown in FIG. 1, the steering sensor 7a is roughly formed in T-shape, and a base edge portion thereof is attached to a drive shaft of the steering motor 3a. Therefore, the steering sensor 7a is swung in horizontal direction in response to the movement of the drive shaft of the steering motor 3a. In addition, detection coils 8a, 9a are respectively attached to tip edge portions of the steering sensor 7a, so that they can detect a position of a magnetic tape (i.e., guiding line) 11 laid on a traveling way 10. Herein, the detection coil 8a is attached to a right edge portion of the steering sensor 7a in forward direction A, while another detection coil 9a is attached to a left edge portion of the steering sensor 7a. Detection of the guiding line 11 is made by use of the electromotive force to be occurred when the coils 8a, 9a move in the magnetic field of the guiding line 11. As known well, such electromotive force becomes smaller as the distance between the coil and guiding line becomes larger. Incidentally, 12 designate rear wheels which are provided in the rear portion of the vehicle such that they can rotate freely.
FIG. 2 is a block diagram showing a diagramatical configuration of a steering angle control unit which is used for the forward travel of the unmanned carrier vehicle 1. In FIG. 2, the electromotive force to be induced by the detection coils 8a, 9a are amplified in amplifiers 13, 14, which outputs are supplied to a differential amplifier 15. Therefore, the differential amplifier 15 outputs a difference voltage (hereinafter, referred to as "deviation signal") between the output voltages of the amplifiers 13, 14. In the present example, when the steering sensor 7a is deviated from the guiding line 11 in leftward direction (see FIG. 5(a)), the output of the differential amplifier 15 becomes negative. On the other hand, when the steering sensor 7a is deviated from the guiding line 11 in rightward direction (see FIG. 5(b)), the output of the differential amplifier 15 becomes positive. For example, in the case where the steering sensor 7a is deviated from the guiding line 11 in rightward direction and the outputs of the amplifiers 13, 14 are respectively at 4 V, 1 V, the output of the differential amplifier 15 (i.e., deviation signal) is at 3 V. Such deviation signal is supplied to both of a differentiation circuit 16 and a proportional circuit 17. The differentiation circuit 16 performs a time-differentiation (i.e., time derivation) on the deviation signal to thereby detect the deviating direction of the vehicle. Then, outputs of the differentiation circuit 16 and proportional circuit 17 are both supplied to an amplifier 18 wherein they are added together. The addition result of the amplifier 18 may indicate that the vehicle is deviated from the guiding line 11 in leftward direction and it is moving in leftward direction, for example. Such addition result is supplied to a drive circuit 19, by which the steering motor 3a is controlled and the steering angle of the front wheel 2 is varied such that the deviation is corrected.
On the other hand, in the case where the unmanned carrier vehicle 1 is traveled in backward direction, the detection operation is switched from the steering sensor 7a to another steering sensor 7b (see FIG. 3) which is fixed in the rear portion of the vehicle 1, so that as shown in FIG. 4, an adder 20 and an amplifier 21 are added to the circuit shown in FIG. 2. The adder 20 adds an output of the amplifier 21, representing a current steering angle, to the output of the amplifier 18 which is produced based on the electromotive force picked up by detection coils 8b, 9b. Then, the addition result of the adder 20 is supplied to the drive circuit 19. Herein, the current steering angle is detected by a steering angle detector DT attached to the drive shaft of the steering motor 3a. Incidentally, different constants for the differentiation circuit 16 and proportional circuit 17 are set respectively for the forward travel and backward travel.
Meanwhile, the above-mentioned steering angle control unit of the unmanned carrier vehicle 1 suffers from the following drawbacks.
(1) Since the non-linear system is used to detect the deviation from the guiding line 11 and then output a steering angle control command for the front wheel 2, some deviation from the guiding line makes the control of the front wheel 2 impossible or it makes the vehicle to be moved in hunting manner. In order to avoid the above-mentioned uncontrollable event etc., the constants of the differentiation circuit 16 and proportional circuit 17 must be adjusted. However, it takes a long time (e.g., three or four days) to adjust the constants.
(2) Even if the constants of the circuits 16, 17 are adjusted as described above, it is required to re-adjust the constants when condition of a floor 10 or kind (or shape) of the unmanned carrier vehicle 1 is changed.
(3) Due to the un-stable elements existed in the control system, it is impossible to raise the traveling velocity of the vehicle.
(4) It is required to partially change the constants of the circuits 16, 17 and circuit configuration in accordance with the forward travel and backward travel respectively, which raises the whole cost of the system.
Meanwhile, FIG. 6 is a block diagram showing a velocity control unit used for the vehicle 1. In FIG. 6, a multiplier 23 multiplies a steering angle signal Sa by a reference velocity signal Vref. Herein, the steering angle signal Sa is outputted from an encoder (not shown) which detects the steering angle of the steering motor 3a. Herein, the polarity of the signal Sa turns to negative when the vehicle is steered in leftward direction, while it turns to positive when the vehicle is steered in rightward direction. In addition, 24 designates a velocity generator which generates a velocity feedback signal Vf having a level corresponding to the number of revolutions of a traveling motor 3b. Further, a control circuit 25 receives the output of the multiplier 23 and velocity feedback signal Vf, thereby outputting a velocity command signal Sc to the traveling motor 3b. FIG. 7 shows an example of the control characteristics of the control circuit 25. In the graph shown in FIG. 7, vertical axis represents the foregoing velocity command signal Sc, while horizontal axis represents the steering angle. As shown in this graph, the unmanned carrier vehicle 1 travels at the reference velocity (e.g., 4 km/h) while the steering angle lies in the range of .+-..theta.. On the other hand, when the steering angle becomes larger than the steering angle +.theta. or becomes lower than -.theta., the vehicle 1 travels at a velocity which is inversely proportional to the steering angle. In short, as the angle by which the vehicle 1 is steered becomes larger, the traveling velocity of the vehicle 1 becomes slower.
When considering the case of the manned vehicle which is driven by the person, the person, who drives the vehicle based on his experiences, decelerates the traveling velocity of the vehicle at first when entering into the curved way, and maintains the traveling velocity at constant while traveling through the curved way, and then accelerates the traveling velocity when the curved way has been almost passed away, by which it is possible to manually carry out the smooth velocity control for the manned vehicle.
However, the unmanned vehicle is designed to carry out the velocity control in response to the steering angle. Therefore, there is a drawback in that the unmanned vehicle cannot carry out the smooth velocity control which can be made by the person. In addition, when entering the curved way having a relatively small turning radius, there is a possibility in that the unmanned vehicle cannot decelerate the traveling velocity sufficiently and therefore it cannot travel the curved way well.
FIG. 8 shows another velocity control unit for the unmanned carrier vehicle. As comparing to the foregoing velocity control unit shown in FIG. 6, the velocity control unit shown in FIG. 8 is characterized by providing a switching circuit 26 which can selectively set one of the predetermined four velocities as the reference velocity in accordance with velocity information. Herein, the velocity information is read from a mark (not shown) located at a position near the guiding line on the traveling way by a mark detector (not shown) attached to the vehicle 1. Then, the reference velocity selected by the switching circuit 26 is supplied to the control circuit 25.
Since the unmanned carrier vehicle 1 requires both of the steering angle control unit and velocity control unit, there is a problem in that the circuit configuration must be complicated. In addition, the unmanned carrier vehicle 1 is designed to travel the curved way at constant velocity which is determined in response to the steering angle control required for the sharpest portion of the curved way, so that there is another problem in that the traveling efficiency cannot be raised.